Try and find one of the old Vantage Point hang on stand. They used a large pin to hang on. I put one up in a very large tulip tree (over 30" circumface) I then used a long ratchet strap for secondary security. The stand worked great until some peice of garbage stole it I would also contact Chippiwa Wedge lock stands, they work great and the owner might be able to help you out with a longer chain set up.
Good luck,

Bmott